We have developed a novel technique for monitoring metabolic competency in female heterozygotes for ornithine transcarbamylase deficiency (OTC). Our method uses mass spectrometry to measure the conversion of 15NH4C1 to (15N) urea and (5-15N) glutamine during a 4 hour period following an oral 15NH4C1 load (25.9 mmol). We found that asymptomatic heterozygotes converted significantly less NH3 nitrogen to urea. Thus, the (15N) urea concentration (mM) in the blood of symptomatic heterzygotes was significantly less than control values at most points. The blood concentration of (5-15N) glutamine (mM) was significantly higher in both asymptomatic and symptomatic heterozygotes than it was in the control subjects. The administration of a test dose of sodium phenylbutyrate to the control group did not affect the rate of (15N) urea formation.